(1) Field of the Invention
This invention relates to a body section radiographic apparatus for repeatedly acquiring fluoroscopic images while synchronously moving a radiation source which emits a beam of radiation to a subject to be imaged and a flat panel detector which acquires fluoroscopic images of the subject, and constructing a sectional images of the subject based on the series of fluoroscopic images obtained. More particularly, the invention relates to a body section radiographic apparatus having a radiation grid disposed between a radiation source and an FPD for transmitting a beam of radiation, and to a noise removing method for use in the body section radiographic apparatus.
(2) Description of the Related Art
Tomographic X-ray apparatus are known as apparatus for acquiring sectional images of subjects using X-rays. Such a tomographic X-ray apparatus serially acquires X-ray fluoroscopic images while synchronously moving an X-ray source and an FPD in opposite directions as opposed to each other across a subject. These X-ray fluoroscopic images are combined to reconstruct, by digital processing, a sectional X-ray image of a desired section of the subject to be displayed on a display device such as a monitor (see Japanese Unexamined Patent Publication No. 2004-236929, for example).
FIG. 13 is a view illustrating a construction of a conventional tomographic X-ray apparatus. A conventional tomographic X-ray apparatus 100 includes an X-ray source 101 for emitting a cone-shaped X-ray beam to a patient M, a sheet like FPD (flat panel detector) 102 for detecting X-rays transmitted through the patient M, a sheet like X-ray grid 103 disposed between the X-ray source 101 and FPD 102 for absorbing scattered X-rays, and a top board 104 for supporting the patient M. The X-ray grid 103 has, arranged therein, a plurality of strip-shaped absorbing foils 103a formed of an X-ray absorbing material. When the X-ray grid 103 is seen as a whole, the plurality of absorbing foils 103a are arranged like slats of a window blind, i.e. parallel to one another and at regular intervals.
The X-ray source 101 and FPD 102 are movable along the direction of the body axis X of the patient M. The sheet like X-ray grid 103 is fixed to the FPD 102. Therefore, the X-ray grid 103 follows movement of the FPD 102 during the serial acquisition of X-ray fluoroscopic images, constantly to prevent scattered X-rays from falling on the FPD 102.
Incidentally, the X-ray grid 103 may have uneven X-ray transmittances at certain parts thereof. In this case, the unevenness of X-ray transmission through the X-ray grid 103 is projected on the FPD 102, to be superimposed on the X-ray fluoroscopic images of the patient M. Then, an X-ray sectional image finally obtained will include granular noise due to the X-ray grid 103. A clear X-ray sectional image cannot be obtained.
In the case of the tomographic X-ray apparatus, the X-ray source 101 and FPD 102 synchronously move in opposite directions as opposed to each other across the patient M. Thus, the position of the X-ray grid 103 relative to the patient M may shift during acquisition of X-ray fluoroscopic images. In that case, the unevenness of transmission through the X-ray grid 103 is appeared as shifting on the X-ray sectional image finally obtained. As a result, the unevenness of X-ray transmission through the X-ray grid 103 cannot be ascertained on the X-ray sectional images.
However, the conventional tomographic X-ray apparatus described above has the following problems. In a particular slice position or section (hereinafter referred to as the grid immobility section MS), there occurs a phenomenon in which the X-ray transmission unevenness of the X-ray grid 103 is not erased from the X-ray sectional image. Moreover, since the X-ray grid 103 is appeared also near the grid immobility section MS, an X-ray sectional image showing a site near the grid immobility section MS has the X-ray transmission unevenness due to the X-ray grid 103 appeared thereon. Thus, a clear X-ray sectional image cannot be obtained.
This grid immobility section MS is determined geometrically from a distance between the X-ray source 101 and FPD 102 (SID: source image distance), a distance between the X-ray source 101 and a reference section MA (SOD: source object distance), and a distance between the X-ray grid 103 and FPD 102 (GID: grid image distance). Generally, values of the above distances cannot be varied, but are constants. Thus, this grid immobility section MS will always appear at a predetermined distance and in a predetermined direction from the reference section MA.
FIG. 14 is a schematic view illustrating how X-ray transmission unevenness due to an X-ray grid in the conventional tomographic X-ray apparatus is projected on an FPD. Point p in the grid immobility section MS is projected on the FPD 102 as moving from point P1 to point P2 and to point P3. When an X-ray sectional image is acquired from this grid immobility section MS, X-ray fluoroscopic images are combined while being shifted so that point P1, point P2 and point P3 may become a fixed point. X-rays having passed through point p in the grid immobility section MS are always transmitted through point gp on the X-ray grid 103 to reach the FPD 102, irrespective of positions of the X-ray source 101 and FPD 102. That is, a shadow of point gp of the X-ray grid 103 is superimposed on point P1, point P2 and point P3 of the FPD 102 corresponding to point p. The same can be said also of point q in the grid immobility section MS. That is, a shadow of point gq of the X-ray grid 103 is superimposed on point Q1, point Q2 and point Q3 of the FPD 102 corresponding to point q.
Incidentally, the X-ray grid 103 has varied X-ray transmittances at certain parts thereof. Supposing, for example, X-ray transmittance at point gp of the X-ray grid 103 is lower than that at point gq, X-rays incident on point P1, point P2 and point P3 will become weaker than X-rays incident on point Q1, point Q2 and point Q3. When an X-ray sectional image is acquired of the grid immobility section MS geometrically determined from the SID, SOD and GID, the X-ray transmission unevenness (particulate noise) of the X-ray grid 103 will be superimposed on the image. For the grid immobility section MS, a method of preventing this particulate noise from being appeared is impossible with the conventional construction.
An X-ray sectional image of the grid immobility section MS acquired with the conventional tomographic X-ray apparatus will have the particulate noise due to the X-ray grid superimposed on the image. Such an image is unsuitable for use in examination. For preventing superimposition of such particulate noise on an X-ray sectional image, it is necessary to acquire X-ray fluoroscopic images serially again after changing the SID, SOD and GID. Then, examination must be conducted twice, which increases exposure to X-rays of the patient. A mechanism is also needed for changing the SID, SOD and GID, making the tomographic X-ray apparatus expensive.
This invention has been made having regard to the state of the art noted above, and its object is to provide a body section radiographic apparatus which is free from radiation transmission unevenness of a radiation grid being appeared on a radiation sectional image irrespective of sections of a subject to be imaged, and a noise removing method for use in the radiographic apparatus.